(1) Field of the Invention
The present invention generally relates to a torsion bar device for a power steering system, and more particularly to a torsion bar device which biases a torsion bar by means of supporting portions so as to regain the original neutral position of the steering wheel after the steering wheel is turned.
(2) Description of the Related Art
A power steering of an automotive vehicle is a power-assisted device which facilitates the turning of a steering wheel by a vehicle operator. In order to achieve the function of the power steering, it is necessary to provide a torsion bar device which stably regains the original neutral position of the steering wheel after the steering wheel is turned.
A rotary valve type power steering system including an input shaft, a torsion bar, and an output shaft is known. The rotation of the steering wheel turned by a vehicle operator is transferred to a rotary valve through the input shaft, the torsion bar and the output shaft. Pressure oil from an oil pump is supplied to a power cylinder, and the supplied oil in the power cylinder is supplied back to the oil pump, in accordance with the rotation of the steering wheel. The turning of the steering wheel by the vehicle operator is facilitated by the power cylinder to which the pressure oil is supplied.
The known power steering system includes a valve mechanism which inherently lowers the rigidity of the steering gear mechanism when the steering wheel is around its original neutral position. The known power steering system has a problem in that the steering gear mechanism becomes less rigid when the steering wheel is around the original neutral position. This problem is more appreciable to the operator during high speed running or slalom running of the automotive vehicle.
Several torsion bar mechanisms for enhancing the rigidity of the steering gear mechanism of the power steering system when the steering wheel is around its original neutral position have been proposed.
For example, Japanese Published Utility Model Application No. 2-45109 discloses a proposed torsion bar mechanism. The proposed mechanism is provided in a rotary valve type power steering system for biasing the torsion bar with an elastic force so as to regain the original neutral position of the steering wheel. The proposed mechanism includes two V-grooves on the periphery of the input shaft, two balls fitted in the holes of the output shaft at positions corresponding to the V-grooves, and an annular spring member arranged on the output shaft for pressing the balls onto the V-grooves so as to regain the original neutral position of the steering wheel after the steering wheel is turned.
However, in the case of the proposed mechanism disclosed in the above mentioned publication, it is necessary that the input and output shafts of the power steering system are modified to arrange the V-grooves, the balls and the spring member on the input and output shafts. In addition, the structure of the proposed mechanism is complicated and the manufacturing processes thereof are time consuming, and the productivity and cost performance of the proposed mechanism is relatively low.
Therefore, in order to adapt the proposed mechanism to the existing production vehicles, the structure of the input and output shafts of the power steering system must be modified. It is very difficult to adapt the proposed mechanism to the production vehicles by simply replacing the torsion bar by the proposed mechanism. The modification of the power steering system makes the productivity and the cost performance lower.